Why does one face appears so warped when UV unwraping?
Ngons tend to cause problems for the UV Unwrap algorithm.  Not always but often enough, it seems random which ones will work and which ones won't.  

You can deal with that particular case by pressing Shift+7 on the number pad so the view aligns with that selected faces normal then press u to bring up the Unwrap menu and then select Project from View

Now the shape should now perfectly match the face.  You'll still likely have to resize it though.